1. Field of the Invention
The present invention relates to a fiber test system for testing optical fibers and, in particular, to a system for testing multiple fibers simultaneously for damage or degradation.
2. Prior Art
While fiber optics have been used for a number of years in the telecommunications industry, the deployment of fiber optic networks has escalated tremendously in recent years in the access networks of telephone companies and cable television companies. Because these optical fibers are often subject to stresses that can lead to breaks, degradation or misalignment of the fibers, which can be devastating to the access network, the telecommunications industry has, out of necessity, developed support systems for testing the optical fibers. These systems, commonly referred to as Remote Fiber Test Systems allow an operator at a central location to perform a test on a fiber or fibers within an optical fiber cable to determine if there is any degradation or damage to the monitored fiber. Field studies indicate that 85 to 90 percent of all cable problems can be located in this manner.
In general, these Remote Fiber Test Systems comprise a Test System Controller (TSC), which is located at a central location with an operator, and a plurality of Remote Test Units (RTUs) which are located in the field. A communications subsystem connects the RTUs to the TSC. The RTUs use the well developed principles of Optical Time Domain Reflectometry (OTDR) to acquire the loss profiles of the monitored fibers and these profiles are then communicated to the TSC for storage, analysis and display of the fiber characteristics.
A block diagram for a typical prior art RTU is shown in FIG. 1. The RTU of FIG. 1 is comprised of three major components; the communications and control module CCM, the OTDR module OTDR and an optical switch means OS for switching between each of a plurality of fibers under test. OTDR module OTDR is comprised of a laser L, a receiver R, a data averaging means DA and a coupler C.
In use, communications and control module CCM triggers laser L to send a stimulating pulse to the fiber under test (one of FUT.sub.1 -FUT.sub.N) through coupler C and optical switch means OS. The stimulated fiber then develops a time dependent Rayleigh Backscatter signal in response to the laser pulse which is returned to receiver R through optical switch means OS and coupler C. Receiver R converts the optical signals to electrical analog signals and samples these signals to convert them to digital signals. This sample and convert process continues until no further backscatter signals are detected. The process thus generates a set of data points wherein the value of each point describes the loss of an incremental segment of the stimulated fiber and the total set of values constitutes a single scan of the fiber. As each of these scans generates digital values, they are passed to the data averaging means DA where they are stored and summed with previously accumulated values. When a specified number of scans are completed, the accumulated values constitute the actual loss profile of the fiber. Once the loss profile of each fiber is acquired, it is available for analysis and/or transmission and another fiber may be tested.
One operational mode of the RTU that has been found to be particularly advantageous is a mode called "surveillance." In the surveillance mode, the RTU is programmed to automatically move switch means OS from fiber to fiber and to acquire a loss profile for each fiber. Each of the newly acquired profiles are examined or compared to the known good profile for the fiber in order to locate those fibers that are either deteriorating or have reached a failure point. When such a fiber is found, a message is sent from the RTU to the Test System Controller, thereby notifying the operator of the condition.
There are at least two major drawbacks to the large scale deployment of these typical RTUs in an extensive access fiber network; the high cost of the RTUs and the relatively slow speed at which the loss profiles are acquired. Further, the use of a mechanical switch means in these typical RTUs causes inherent unreliability. The present invention overcomes these drawbacks by providing significant cost reductions, increasing surveillance scan speed by at least a factor two over conventional systems and eliminating the need for a mechanical switch means.